1. Field of the Invention
The present invention relates to a nickel ultrafine powder to be used for internal electrodes of multilayered ceramic capacitors or electrodes of secondary batteries, fuel cells or the like.
2. Description of the Related Art
Nickel ultrafine powder is attracting attention as a material to form internal electrodes of laminated ceramic capacitors, porous electrodes of hydrogen-nickel secondary batteries, hollow porous electrodes for fuel cells (which provide electric energy through electrochemical oxidation of fuel), and electrodes of a variety of electronic parts.
The following description is concentrated on internal electrodes of laminated ceramic capacitors.
A laminated ceramic capacitor is composed of layers of ceramic dielectric (such as titanium oxide, barium titanate, or complex perovskite) and layers of metal internal electrodes which are laminated alternately under pressure and integrally fired. Today, it finds rapidly increasing use as an electronic part. It is decreasing in size and increasing in capacity, with its internal electrodes becoming thinner, to meet demand for electronic machines and equipment of higher performance. It is conventionally provided with palladium internal electrodes, but they are being replaced recently by nickel internal electrodes which are comparatively inexpensive and yet highly reliable.
Japanese Patent Laid-open No. 136910/1989 discloses a technique for producing by wet process a nickel powder having a purity higher than 99% and a particle diameter of 0.1-0.3 .mu.m. However, it does not mention actually making the powder into paste and electrodes for electronic parts. The present inventors found that conventional nickel powder made by wet process suffers the disadvantage of greatly changing in volume at the time of firing, causing continual delamination and/or cracking to the laminate. A conceivable reason for this is that the wet process proceeds at too low a temperature (e.g., lower than 100.degree. C) for sufficient crystal growth, giving rise to agglomeration with fine primary particles, which are liable to over-sintering or marked volume change during firing.
Japanese Patent Laid-open No. 80007/1989 discloses a nickel powder having an average particle diameter of 1.0 .mu.m and a purity of 99.9% which is to be used as paste for electrodes of ceramic capacitors. This paste is incorporated with carbide powder to prevent cracking and/or delamination at the time of firing. However, nothing is mentioned about the characteristics of the nickel powder itself which affect cracking and/or delamination.
Japanese Patent Laid-open No. 246001/1996 discloses a nickel ultrafine powder having an average particle diameter of 0.1-1.0 .mu.m and a purity higher than 99.5 wt %, which is used for laminated ceramic capacitors. It mentions that the powder has a particle size distribution such that the geometric standard deviation is lower than 2.0. This implies that if its average particle diameter is 0.4 .mu.m, the powder may contain coarse particles larger than 1 .mu.m up to about 8% in terms of number-size distribution. In fact, the presence of coarse particles up to several percent is permitted in Examples.
Production of laminated ceramic capacitors needs techniques for preventing cracking and/or delamination at the time of firing, making internal electrodes thinner, rendering capacitors smaller in size and higher in capacity, and eliminating defectives due to short circuits across electrodes.